As a foreword, it should be recalled that a connecting rod of an internal combustion engine is associated on the side of its foot with the bearing of a combustion piston and on the side of its head with the bearing of a crankshaft. These two bearings generally have parallel axes. As shown, respectively, in FIGS. 1A and 1B, the function of the rod is to transmit the translation movement of the piston from a “top dead center” to a “bottom dead center” as the crankshaft rotates. The rod also helps maintain the angular position of the piston in line with the translation axis of the latter.
Several solutions for adjusting the compression ratio and/or displacement of an internal combustion engine are known in the state of the art.
It should also be recalled that the compression ratio of an internal combustion engine, often referred to as the compression rate, corresponds to the ratio of the volume of the combustion chamber when the piston is at the bottom dead center to the volume of the combustion chamber when the piston is at its top dead center. Everything being equal, the length of the rod determines the compression ratio of the engine.
It is generally agreed that adapting the compression rate of an engine to its load enables a great increase in the engine fuel efficiency. For example, designers sometimes seek to vary the compression ratio between 12 when there is no load and 8 at full load.
For a four-stroke engine, it should be recalled that one complete engine cycle comprises a fresh gas intake cycle followed by a compression cycle, a combustion-expansion cycle, and, lastly, an exhaust cycle. These cycles are of reasonably equal extensions, distributed over 720° of the crankshaft rotation.
The engine load can thus be defined as the constant pressure exerted on the piston crown during the combustion expansion part of an engine cycle (with pressure on the piston crown during the complementary part of the cycle being considered as nil), which corresponds to a torque equal to that developed by the engine over a complete cycle. This pressure reaches a maximum of 10 bars for a current naturally-aspirated engine, and can rise jointly to 20 or 30 bars for a supercharged engine.
Displacement, on its part, corresponds to the volume created by the movement of the piston in the master cylinder from the top dead center to the bottom dead center. Variable displacement is achieved by varying the stroke of the piston in the cylinder. Displacement is not influenced by the length of the connecting rod. The displacement variation must be of a high amplitude for it to have any significant effect on fuel efficiency, and this is technologically challenging to implement.
U.S. Pat. No. 4,111,164 therefore aims to vary engine displacement based on the load applied to it. This document discloses a rod consisting of a spring associated with a hydraulic chamber such that the piston is rigidly coupled to the crankshaft of the engine when the latter is not loaded; and elastically coupling the piston to the crankshaft when the engine is under a heavy load. For this second situation where the load is heavy, the connecting rod acts as a shock absorber, compressing and expanding depending on the pressure at each instance in the engine cycle. U.S. Pat. No. 4,111,164 thus discloses a constant displacement with the load during the intake cycle, while the displacement increases during the combustion cycle, when the load increases. However, the combustion forces partly absorbed in the hydraulic chamber of the rod are not returned, which makes the solution particularly inefficient.
This solution does not therefore enable adjustments in the compression ratio depending on the load applied during one or series of engine cycles. The behavior of this rod is particularly sensitive to the engine speed. The solution proposed in U.S. Pat. No. 4,111,164 further leads to intensely solicit the mechanical components of the connecting rod (spring, hydraulic chamber) during operation of the engine, which accelerates their wear out and reduces the reliability of the system.
Furthermore, the hydraulic chamber of the solution presented in U.S. Pat. No. 4,111,164 is particularly sensitive to temperature changes in the hydraulic fluid, and this, in combination with the sensitivity to the engine speed, makes it very difficult to predict the behavior of the rod.
Document R0111863 describes an internal combustion engine made up of a mobile upper block and a lower block fixed to the vehicle chassis. The upper block is free to pivot on a lateral axis linking the upper block to the lower block. When the engine load increases, the effective average pressure in the cylinder increases and causes a movement of the upper block around the lateral axis. A cylinder volume is thus added to the volume of the combustion chamber, thereby causing a reduction in the compression ratio.
The solution offered in this document requires the design and manufacture of an articulated engine block, which corresponds to none of the standard internal combustion engine designs, which all have a fixed engine block. This requires a complete redesigning of most engine-chassis interface components of the vehicle. Therefore, any components connected to the upper part of the engine (air or fuel intake line, exhaust line, distribution, etc.) must be adapted to tolerate the mobility of the upper part of the engine.
Other documents, such as WO 2013/092364, describe controlled length connecting rods that enable a fixed compression ratio in the internal combustion engine (without affecting the displacement). These solutions require an active rod-length steering system controlled through an external command system (hydraulic piston, electric engine, etc.). The external command systems are generally complex and lead to energy losses, in addition to being unreliable. Furthermore, compression ratio control is not continuous and the accessible value range is often too limited. This is especially the case with the solution proposed in the document cited above, which provides only two different rod lengths.